The present invention relates to multi-track optical data recording and readout with magnification and skew compensation.
All optical recording systems incorporate servo controls with sensors for tracking offset, data clock, and clock synchronization derived from recorded data marks and/or preformatted track patterns. However, implementation of such sensors involves tradeoffs between recording areas dedicated to the servo function versus restrictions on data encoding formats and degradation in sensor accuracy and servo performance.
For multichannel recording, it is recognized that a single preformatted track may be used for track and clock sensing for multiple data tracks (see U.S. Pat. No. 4,283,777).
It is also recognized that a global tracking error signal for a band of data tracks can be generated by simultaneously sensing the two edges of the data band, visible because of a guard space left between neighboring data (see U.S. Pat. No. 5,989,671).
Multichannel optical tape readout requires servo operations in addition to the usual tracking, focus, clock, and synchronization. The additional error sources are magnification and skew.
Magnification error causes data tracks to be mapped onto a data detector surface with a pitch that is smaller or larger than nominal. The problem of magnification error is set forth in U.S. Pat. No. 4,969,137 (Hitachi) which rotates an array of recording spots at the media surface so that the spot spacing projected in the media scanning direction matches the desired track pitch. U.S. Pat. No. 5,729,512 discloses a multichannel optical disk readout system with sensors to measure tracking offset and estimate the magnification error for subsequent compensation.
Skew error is an offset in clock synchronization from one data track to the next. Skew error is caused by a relative rotation of the data band mapped onto the detector surface.
Problems associated with magnification and skew errors are most severe during data readout. Deviation from nominal magnification and skew during recording will not interfere with the recording process unless the errors are so great as to cause tracks to overlap. But during readout, errors much less than 1 xcexcm in the in-track or cross-track directions could severely degrade signal quality. Furthermore, the net error during readout is a combination of deviations in the recording system, readout system (possibly a different system), and the media.
Prior art teaches that misalignments (including tracking, focusing, and magnification errors) can be detected by sensors that examine recorded data tracks or utilize light reflected from the data regions of the recording medium. However, effective operation of such sensors imposes restrictions on the recorded data patterns such as track pitch and modulation code and on the media characteristics such as reflective phase shift. For optimal data recording, it is thus desirable to decouple alignment error sensing from the data-bearing tracks.
According to the present invention, co-written control tracks are written on either side of the data band at the time of recording. During readout, independent tracking position error signals and data clock signals are extracted from each control track. The track position of each track in the data band is inferred as a linear interpolation between the two control track positions. The clock offset for decoding each track is inferred as a linear interpolation between the clock offsets measured for the two control tracks.
It is therefore an object of the present invention to effectively control magnification and skew error during data readout and respond to the current state of the recorded data pattern.
This object is achieved by a method for optical data recording and readout, comprising:
a) scanning a blank region of an optical recording medium while recording a data band including a plurality of adjacent data tracks;
b) synchronizing the recording on all tracks to a common data clock;
c) simultaneously recording control tracks with the band of data tracks, each control track having a well-defined track center and a repetitive pattern of marks that is synchronized to the data clock;
d) optically sensing the cross-scan positions and clock offsets of at least one control track on either side of the data band;
e) interpolating between the control track cross-scan positions to predict the cross-scan positions of each data track and compensate for magnification errors;
f) interpolating between the control track clock offsets to predict the clock offset of each data track and compensate for skew errors; and
g) simultaneously reading and decoding information recorded on all tracks of the data band.